1. Field of the Invention
This invention relates to underdrains in gravity filters and, more particularly, to caps for underdrain distributors, such as filter underdrain blocks.
2. Description of the Prior Art
Gravity filters frequently employ underdrain systems for controlling and channeling effluent away from the filter bed and also for distributing backwash fluids into the filter bed. Various underdrain systems are shown and described in U.S. Pat. Nos. 3,840,117; 4,065,391; and 5,019,259. U.S. Pat. No. 5,108,627 to Berkebile, et al., incorporated herein by reference, describes an underdrain system having several filter underdrain blocks arranged end-to-end in parallel, adjacent rows across a filter bottom. The underdrain blocks provide the appropriate conduits and apertures for receiving effluent from the filter bed and discharging backwash fluids into the filter bed. The underdrain blocks have several apertures in a top wall of the blocks for this purpose. The apertures are in communication with upper secondary conduits where mixing of backwash air and water may take place prior to being introduced into the filter bed.
It has been found desirable to employ larger apertures in the top wall of the blocks so that the apertures will not clog and inhibit flow to and from the filter bed. The problem with the larger apertures is penetration of fine grain filter media, such as sand, anthracite or granular activated carbon ("GAC"), through the apertures and into the underdrain blocks. This eventually clogs the underdrains, and the loss of filter media hinders the filtering capabilities of the bed. The conventional remedy for this problem is employment of a support gravel barrier between the fine grain filter media and the underdrain blocks. However, the support gravel barrier necessitates the expensive and time-consuming installation of several discrete, layers of gravel. The support barrier typically also consumes 12 inches of filter space, which could better be used to increase the filtration capabilities of existing gravity filters or to reduce the size of newly designed gravity filters. Other problems related to employment of a gravel support barrier are discussed in the above-identified related patent application, Ser. No. 07/679,947.
Several attempts have been made in the past to produce underdrain systems which do not require support gravel barriers. For example, U.S. Pat. No. 5,068,034 to Walter discloses an underdrain lateral having three vertically stacked chambers with a nozzle having a threaded stem engaged on a top wall of the lateral. A frusto-conical upper member of the nozzle contains slotted distribution orifices for passing backwash fluids to the filter bed without admitting granular material into the lateral. A problem with this system is that backwash air and water are not intermixed until after each enters the nozzle. Thus, the scouring effect of air in the filter bed is not maximized.
A publication by C.P.C. Engineering Corporation, dated 1991, describes "Triton" underdrain systems that have a rounded screen overlying an internal flow distribution assembly. The Triton underdrain is inhibited from properly distributing backwash fluids because it cannot practically be installed at true level. Typical variations in underdrains deviate as much as 1/8' from level across the length of the underdrain. As a result, incoming backwash gas collects at the high end of the underdrain, and distribution into the filter bed is uneven. This problem is enhanced by the fact that the channels and the underlying plenum in the Triton underdrain are in fluid communication across the full length of each underdrain distributor. The only metering of gas which occurs is between the underlying plenum and the apertures in the channels. The openings in the overlying screen do not provide sufficient head loss to generate any significant metering effect. Additionally, there can be little or no intermixing of backwash air and water prior to introduction to the filter bed.
Finally, U.S. Pat. No. 5,019,259 to Hambley discloses an underdrain apparatus having folded plates and apertured grids extending across troughs formed between the plates. The apertures in the grids may be fitted with individual strainers to act as fine media retainers. Each strainer has a slotted, frusto-conical body and is threadingly engaged in the aperture. However, each strainer is in fluid communication with just one aperture and receives partially intermixed backwash air and water from a secondary chamber defined by each trough. The secondary chambers are in fluid communication across the entire underdrain, as with the Triton underdrain described above.
Thus, it is an object of the present invention to provide a capped underdrain distributor which eliminates the need for support gravel in the overlying filter bed, while maximizing the distribution of backwash air and water across the entire surface of the underdrain. It is a further object to promote intermixing of backwash air and backwash water prior to introduction into the filter bed for improved scouring and cleansing of the filter media. It is a still further object to provide a capped underdrain distributor which resists fouling and which is easy to install. Finally, it is an object of the present invention to provide a capped underdrain distributor which creates sufficient head loss that backwash fluid distribution is maximized, without needlessly increasing the energy requirements for backwash operations.